Polarimetric radar mapping processes have been used, via aircraft and satellites, to determine features of a geographical region of interest with a high degree of resolution. In particular, such features as the biomass or the soil moisture content of the region can be determined from polarized radar signals transmitted toward and received from the region. Typically at least two signals having distinct predetermined signal polarizations are transmitted and received. The received signals which are deflected or scattered from the region are analyzed according to their polarization, amplitude and phase. To properly analyze a received signal, a polarimetric radar signal mapping process must calibrate signal derived data to account for various signal distortions. For example, there are imbalances between the circuits or channels transmitting distinct polarized signals. That is, there can be unintended amplitude and/or phase differentials between these channels resulting in two signals of distinct polarization being transmitted with unintended imbalances in their amplitudes and/or phases. Further, similar channel imbalances can occur upon reception of received signals scattered from the region of interest. Additional signal distortions can result from the leakage or cross-talk of signals having one polarization into circuitry for the transmission or reception of signals having a different polarization.
For the above-mentioned distortions, there are well known equations which provide functional relationships between: (a) measurements of the received scattered signals and (b) calibration coefficients or variables used to reduce the above distortions. Moreover, there have been various prior art mapping processes for determining the cross-talk and channel imbalance calibration coefficients for calibrating the received scattered signals. Such processes incorporate one or a combination of two basic approaches: (1) receiving signals from artificial (i.e., man-made) ground-based calibration devices (i.e., calibrators) with known scattering or transmission properties, and (2) receiving signals from natural scatterers with assume signal scattering properties.
The first approach has the advantage that calibration accuracy is limited only by the precision with which the ground-based calibrators can be built, maintained, and oriented. However, such mapping processes are constrained from a practical standpoint by the need to locate the calibrators in areas where the calibrators can be clearly detected. That is, in areas having low signal scattering. Such areas, of course, can be difficult to provide in certain regions of interest. Further, a plurality of such calibrators can be required within each scene for which calibration is required, thereby entailing increased ground support.
Mapping processes using a combination of the above-mentioned two approaches have been proposed to reduce the number of required ground-based artificial calibrators. Such processes have been developed for calibrating data using a single artificial ground-based calibrator together with selected observations of natural signal scatter. However, such processes typically assume reciprocity of the radar system. That is, the transmit imbalance and cross-talk are assumed to be equal to the equivalent receive values. Moreover, such processes also assume the cross-talk terms are small so that the calibration computations can be linearized. with this latter assumption, the cross-talk terms can be determined using signal scattering measurements, but the artificial calibrator is still required to obtain the channel imbalance calibration coefficients. Further, such processes require an initial estimate of the calibration coefficients and converges to the final values through an interactive process. Alternatively, mapping processes using both the above-mentioned approaches have also been developed that eliminate the reciprocity assumption. However, in either case the channel imbalance calibration coefficients are valid only in a local area of the artificial calibration. In fact, the channel imbalance calibration coefficients can vary by 25% or more across a region of interest having a transmitted signal incidence angle range of 25.degree. which correspond to an amplitude variation of 1.9 dB. An array of ground-based calibrators is thus required to calibrate most regions of interest.
In yet another polarimetric signal mapping process that also uses a single ground-based artificial calibrator, the artificial calibrator is used to obtain all of the above calibration coefficients. However, the process is again limited in that the validity of the calibration is restricted to the vicinity of the calibrator. In addition, this process also assumes that the cross-talk calibration coefficients all have the same value, an assumption that is not satisfied by circuitry of many polarimetric radar signal mapping systems, including the current JPL AirSAR polarimetric radar transmitting and receiving system.
Also, for signals at frequencies below approximately 1 GHz that must travel through the ionosphere, Faraday rotations in the angle of a polarized radar signal occur which can distort the signal. The signal is distorted both during transmission to the region of interest and following the scattering of the signal from the region. Thus, such low frequencies have been avoided in prior art polarimetric signal mapping processes due to inadequate techniques in compensating for such Faraday rotations. However, polarimetric radar signals at such low frequencies are useful in penetrating deeply into areas with substantial vegetation. Thus, it would be advantageous for a spaceborne polarimetric radar signal mapping process to be able to use such low frequency signals for biomass or other applications and perform signal calibration to reduce the Faraday rotation angle distortion.
The present invention eliminates many of the abovementioned drawbacks inherent in polarimetric signal calibration processes.